The origin, function and fate of germinal center cells in peripheral lymphoid tissue will be examined. Transfers of labeled cells such as spleen and bursa in histocompatible chickens, immune lymph node in rabbits and bone marrow in inbred mice will be employed. Particular attention will be given to the possible influence of specific antigen and antigen-antibody complexes on the homing of lymphoid cells. The memory function of thymus and bone marrow-derived lymphocytes (T- and B- cells) and the cell interactions during the secondary immune response in vitro will be analyzed. Reconstitution of the immune responsiveness of mouse or chicken cells, which have been rendered incompetent after treatment with antisera and C (specific anti-thymus or anti-bursa: chickens; anti-O and anti-Ig: mouse), by addition of a variety of immune and normal cells will be studied. Three experimental models will be used to obtain further information on the nature of the antigen-specific receptor of immune T-cells. In one, the agammaglobulinemic, bursectomized chicken, in vivo inhibition of the delayed hypersensitivity response by anti-Ig sera, labeling of cell surfaces by I125, and proliferative responses of spleen cells will be studied. In the second, the proliferative response of immune mouse lymph node cells, and in the third, the mixed lymphocyte reaction between rabbit thymus cells, the capacity to interfere with cell bound immunity in vitro of a variety of anti-Ig of different specificities, including antiallotypes and anti-idiotypes, will be tested. Further experiments planned concern: 1) the possibility that T-cells are being competed for in the phenomenon of antigenic competition; 2) the breaking of tolerance by exposure of cells in vitro to antigenantibody complexes; 3) the sites of synthesis of C'2 and C'8 in vitro.